Apparatus for deposition by flame hydrolysis

An outside chemical vapor deposition apparatus is disclosed for depositing porous glass-forming material onto a target rod to form a cylindrical body, with substantially higher efficiency than could prior apparatus. The apparatus includes two separate burner arrays, one array optimized for depositing the material during an initial stage of the process, when the cylindrical body has a relatively small diameter, and the other array optimized for depositing the material during a later stage of the process, when the cylindrical body has a relatively large diameter. In addition, each burner array can include a plurality of burners, with each burner azimuthally angled relative to the apparatus' air-flow axis, and with adjacent burners angled in opposite directions relative to that axis, to minimize the density gradient within the deposited porous material.